The released was made.
https://github.com/rbenv/ruby-build/releases/tag/v20181018
You can check the commits.
https://github.com/rbenv/ruby-build/compare/v20181018...master
The problem is that the description of the release is wrong. It says:
When it should say
What I see is that the v20181018 tag (the last release) has MRI 2.5.2.
The commits since that release v20181018...master did add MRI 2.5.3, but a new tag/release wasn't added.
Agreed, I am running v20181018
and do not see 2.5.3 that is in master but not released:

Yes, sorry my mistake. You are right.
I downloaded the released version and the file share/ruby-build/2.5.3 wasn't packaged.
Most helpful comment
What I see is that the v20181018 tag (the last release) has MRI 2.5.2.
The commits since that release v20181018...master did add MRI 2.5.3, but a new tag/release wasn't added.