Moving from V2.0 to V3.0

Thanks @stephen ollman for raising this issue.

The version numbers are a bit misleading, but 3.0.x is based on 8.x-1.x.
If you are using 2.0.x then you should not be updating to 3.0.x.

8.x-1.x ==> 3.0.x, is basically the original version of the module, based on the port from D7, where a single field CSS classes is added to blocks' configuration form:
https://git.drupalcode.org/project/block_class/-/blob/3.0.x/config/schem...

2.0.x is a new version of the module, with quite a lot of new features, such as support for CSS attributes, bulk operations on classes, etc...
https://git.drupalcode.org/project/block_class/-/blob/2.0.x/config/schem...

Note that currently 2.0.x has quite a few issues and is still a work in progress:
If you look at the issue queue you should see quite a few issues related with autocompletion and various other PHP related issues.
3.0.x has a much smaller scope of features and has existed for much longer, therefore it is probably much more stable.

We plan on updating both branches documentation and more generally the project page to provide more information on supported branches.

Not sure if I was able to answer your questions, but feel free to let us know if you have more or if you would need more information on any of these branches or the project in general, we would surely be glad to help.
Thanks in advance!

Is there any risk in moving from Version 2.0 to 3.0 with a standard composer update?

For follow up readers in short: Yes there is. And the chance is high that it will break your site and you will loose possibly a lot of settings because 3.0 misses a lot of features from 2.0. Read on here: #3468976: Untangle branch confusion and document missing features in 3.x

The release policy and versioning used at this project is - well nicely spoken - very confusing and should be documented if not changed.

e.g. Hide branch 3 or create branch 4 to place the code of branch 2 in there and make it recommended and remove recommended tag from 3. Since it misses some expected features and users can run into broken sites when they follow update recommendation notices from Drupal core or upgrade status and similar projects.

If you are using 2.0.x then you should not be updating to 3.0.x.

As the quickest fix for now: This should be placed on top of the project page. I fully understand from own experience if time is limited. But then, give me temp access to do it. It just takes 1-2 minutes.

Thanks everyone for all the help and explanations!

Looks like this has been addressed in:
#3468976-17: Untangle branch confusion and document missing features in 3.x

with the 3.x and 4.x branches both having stable releases with automated testing pipelines in place.👌

Since there has not been any more activity on this issue for more than a year, it is probably safe to assume it could be marked as Fixed, for now.

Feel free to let us know if you have any questions or concerns on any of the recent code changes or the project in general, we would be glad to help. 😊
Thanks again very much!