When default scaffolding is created with the 1.7.1 --plusbeta version of the generator and select "Create a subfolder", the "teams" folder should be present as described in this article:
https://docs.microsoft.com/en-us/sharepoint/dev/spfx/web-parts/get-started/using-web-part-as-ms-teams-tab
If you create the scaffolding with 1.7.1 --plusbeta, and select "Create a subfolder" the "teams" folder is absent.
1) Make sure SPFx v1.7.1 is installed
2) yo @microsoft/sharepoint --plusbeta --skip-install
3) During the initial setup wizart, select "Create a subfolder"
4) Observe that the teams folder is absent
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What's your installed environment look like (_Node, NPM, Yeoman versions_)? Just did the same with v1.7.1 and it is present... so I can't repro...

There was an issue with v1.7.0 where it was adding the teams subfolder all the time, regardless if you used --plusbeta or not. The v1.7.1 fixed this so it should only appear when you include the beta flag.
Here is my environment:

I noticed you are on a mac and I am on windows. I don't have a mac handy to test if that's the difference?
And for comparison, I downgraded to v1.7.0 and ran the generator with --plusbeta again:

It's not a Mac / Windows thing... I have the same expected behavior on Windows:

So it's something else local to your machine I think... still can't repro your experience...
BTW... as I said in my last post, you will always see the teams folder get created in v1.7.0 because of a bug in the generator. Regardless of the presence of the --plusbeta switch, the v1.7.0 generator will ALWAYS add that folder. v1.7.1 fixed this behavior so you only see it when adding the flag.
Thanks for your help on this @andrewconnell! I think I know whats wrong. The teams folder is missing when I select "Create a subfolder" in the initial wizard. I also don't get the question which determines the isDomainIsolated setting

However, if I select "Use the current folder", I get the teams folder as well as the isDomainIsolated question:

Are you able to reproduce this?
Good find... yes, I can confirm that when you select "create a subfolder with a solution name", it's like the --plusbeta is not respected. Will raise this with engineering too...
Although the package.json does have all the 1.7.1-plusbeta versions of the SPFx packages. That's what I used to confirm that I wasn't going mad and was indeed using the --plusbeta flag.
Clearly a bug, which we'll be checking to fix for the 1.8 release. Thx @vman for reporting.
Fixed in 1.8, which is coming out soon.
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Good find... yes, I can confirm that when you select "create a subfolder with a solution name", it's like the
--plusbetais not respected. Will raise this with engineering too...