Pressing the backspace key results in a warning... warn: command 'markdown.extension.onBackspaceKey' not found - and it doesn't backspace. See attached image.

That's strange. I cannot reproduce.

It seems that you have other markdown extensions installed. Could you please disable the other extensions and then test again.
I bet this is also fixed in v0.7.7
I solved this by redefining the key shortcut for the markdown extension of markdown.extension.onBackspaceKey to Backspace.
🤔 still no idea about why
I was accidentally messing with the F# keys, but I am not sure which one triggered the error.
Happening here on Mac OS X. Just installed the extension. Enter and Backspace give similar warnings.
command 'markdown.extension.onEnterKey' not found
command 'markdown.extension.onBackspaceKey' not found
Basically, all of these:
Developer console shows:
Cannot find module 'markdown-it-task-lists' (at /Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/out/vs/workbench/node/extensionHostProcess.js:297:87)
Edit: Manually running npm install markdown-it-task-lists --production in the extension folder removes the console error, but doesn't solve the keyboard problem.
Edit2: A couple times closing and opening VSCode and it's working now...
🤣 Thanks for your report.
One common reason is that the extension requires some time to load when you open a Markdown file for the first time. For this case, wait 2 seconds after you open a md file.
Another possible reason is the extension files are broken. Then you can try to reinstall the ext.
Happening to me now. Sometimes, pressing Enter causes command 'markdown.extension.onEnterKey' not found, sometimes it just takes _very_ long to be executed (10-20 seconds).
I've tried waiting after VSCode fully loads and opening MD file only then, no help.
My installation:
Extension|Author (truncated)|Version
---|---|---
spellright|ban|2.1.10
emojisense|bie|0.3.0
markdown-preview-github-styles|bie|0.1.0
npm-intellisense|chr|1.3.0
path-intellisense|chr|1.4.2
vscode-markdownlint|Dav|0.13.0
ghlink|dt|0.0.6
gitlens|eam|7.5.7
EditorConfig|Edi|0.12.0
tslint|eg2|1.0.24
vscode-npm-script|eg2|0.3.3
LogFileHighlighter|emi|1.2.0
auto-rename-tag|for|0.0.15
rest-client|hum|0.16.2
vscode-github|Kni|0.26.0
goto-last-edit-location|kri|0.2.1
git-tree-compare|let|1.1.4
terraform|mau|0.0.20
tgit-cmds|mbi|1.1.1
dotenv|mik|1.0.0
debugger-for-chrome|msj|4.1.0
vscode-docker|Pet|0.0.23
code-settings-sync|Sha|2.8.7
vscode-terminal-here|Tyr|0.1.1
es-quotes|vil|0.2.6
quokka-vscode|Wal|1.0.94
wallaby-vscode|Wal|1.0.75
local-history|xyz|1.6.2
markdown-all-in-one|yzh|1.0.3
The extension is loaded when you open a MD file the first time.
So wait 2 seconds after opening a MD file (not after vscode launches).
I'm considering whether I need to add a status bar item to indicate that this extension is ready...
Do you see any error in the console (menubar>help>toggle developer tools)?
Even when I open a MD file and wait for 10 seconds, the Enter key is extremely unresponsive (10-20 seconds reaction time). There are no errors in the dev tools related to this extension.
I suspect it's some other extension; I've tried everything on a clean installation of VSCode Insiders with just this plugin installed and all is fine.
Any idea how to debug what could be competing for the Enter key?
The issue seems to be happening with the KnisterPeter/vscode-github extension enabled, I'll post an issue there. UPDATE: https://github.com/KnisterPeter/vscode-github/issues/298
I have an idea.
Open your extension folder (usually C:\Users\[username]\.vscode-insiders\extensions) with vscode
Use global search in the left activity bar
Find which package.json contains "key": "enter"
It seems that extension doesn't override Enter key... Anyway, leave this issue to that author 😂
so, i have this issue as well. removing this extension fixes the issue. i tried manually overriding the keymap to Backspace but didnt seem to change anything. it was already mapped. I looked for other extensions that use the backspace key as you mentioned, and only this extension seems to do that for me
❯ ag 'key": "enter' ~/.vscode-insiders/extensions
~/.vscode-insiders/extensions/yzhang.markdown-all-in-one-1.0.5/package.json
80:6: "key": "enter",
1 matches
1 files contained matches
4326 files searched
21388767 bytes searched
0.452680 seconds
my extensions:
auiworks.amvim-1.24.0
bierner.markdown-checkbox-0.1.1
bierner.markdown-footnotes-0.0.4
bierner.markdown-mermaid-0.1.1
bungcip.better-toml-0.3.2
christian-kohler.npm-intellisense-1.3.0
christian-kohler.path-intellisense-1.4.2
Connorcpu.vsc-rustfmt-0.1.0
DavidAnson.vscode-markdownlint-0.13.0
dbaeumer.vscode-eslint-1.4.6
dotcypress.vscode-cargo-test-0.0.1
DSKWRK.vscode-generate-getter-setter-0.4.2
eamodio.gitlens-8.0.2
eg2.tslint-1.0.28
eg2.vscode-npm-script-0.3.3
Equinusocio.vsc-material-theme-1.4.2
esbenp.prettier-vscode-1.1.3
fatihacet.gitlab-workflow-0.4.3
GregorBiswanger.json2ts-0.0.6
kevinmcgowan.typescriptimport-1.17.0
leizongmin.node-module-intellisense-1.5.0
lextudio.restructuredtext-50.0.0
mkloubert.vscode-git-notify-0.4.2
ms-python.python-2018.1.0
mtxr.sqltools-0.12.0
njpwerner.autodocstring-0.1.4
PeterJausovec.vscode-docker-0.0.24
PKief.material-icon-theme-3.2.3
pmneo.tsimporter-1.2.14
rbbit.typescript-hero-2.2.0
richie5um2.vscode-sort-json-1.11.0
robertohuertasm.vscode-icons-7.20.0
rust-lang.rust-0.3.2
ryanluker.vscode-coverage-gutters-1.3.1
searKing.preview-vscode-1.4.2
tushortz.python-extended-snippets-0.0.1
vadimcn.vscode-lldb-0.7.5
wholroyd.jinja-0.0.8
xabikos.javascriptsnippets-1.5.0
yzane.markdown-pdf-0.1.7
yzhang.markdown-all-in-one-1.0.5
ZakCodes.rust-snippets-0.0.1
zhuangtongfa.material-theme-2.12.4
I experience the problem with the backspace key upon upgrading to VS Code 1.21.0 (February). I tried adding preference:
"markdown.extension.onBackspaceKey": "Backspace"
But am warned that markdown.extension.onBackspaceKey is not a valid preference.
I uninstalled the following extensions and my problem disappeared:

I just reloaded the "Markdown All in One" extension and the error went away.
You can find more details in #103
@neilsustc
You can find more details in #103
It did not help me. These 4 files already have vscode.markdown-language-features string.
Are you using the latest vscode (v1.21.0)?
You can just try right click and delete the key-bind~
@smilelight Deleting the keybindings works for me. Thanks!
@fvcproductions thx bro
Well, In my case I was using the 'Markdown All in One' extension, and I had the same problem, for instance an error as follows:
"command markdown onEnterKey not found"
I could solved it by reloading the extension, Go to Extension Tab, search your extension and click on the 'Reload' blue button, but you could reinstall it again.

I am experiencing the same problem, this problem was solve when I reload Markdown All in One.
I experience still this problem.
This issue is too long ago. Feel free to open a new one (with some information).
Most helpful comment
I just reloaded the "Markdown All in One" extension and the error went away.