TYPO3 Core

I would love to use the simplified classes of Bootstrap.
For Migrations see:
https://getbootstrap.com/docs/4.0/migration/
http://upgrade-bootstrap.bootply.com/

Bootstrap is almost up to 4.3, and 5 is in heavy development. Bootstrap 5 will support IE11 and also removes jQuery as a dependency. I'd suggest developing with BS 5 for the TYPO3 v10 backend (as soon 5 is available), rather than switching from 3 to 4 first.

I would generally agree though afaik it was said that the migration from 4-5 should be easier than from 3-4 which would make the transition a bit easier when first migrating to version 4.
It also kind of depends on the release timing of bootstrap 5 in correlation to TYPO3 v10

If you know when Bootstrap 5 will be available (which in general @mdo does not spoil), we can jump on Bootstrap 5.

However, I think we need to update to Bootstrap 4 as soon as possible. It's actually not as easy, as we still have some JS-based bootstrap plugins that we also need to update. (DateTimePicker). So the first step is to ensure that we can upgrade to Bootstrap 4 via yarn.

Once this is done, we can continue with changing all classes in the packagse.

I will try and prepare a patch once this lands in the bootstrap-slider repository:
https://github.com/seiyria/bootstrap-slider/pull/856

I agree with Benni Mack that the bootstrap version for the backend should be updated as soon as possible.

All sites for goverments or public administrations need to pass pen tests not only for the frontend output but often for the backend too. For instance pen tests for the backend are inevitable when the backend login page is accessable for the public.

One main requirement of these tests are that all frameworks should use only actual versions. In our case the last pen test of a TYPO3 9 installation ended with a negative result simply because of the outdated bootstrap plugin used in the backend of TYPO3 9 LTS. I assume this could become an increasing problem for all companies which build sites for public administrations, especially when TYPO3 10 uses the ooutdated bootstrap version too.

as far as I have understood the datepicker has become this one: https://tempusdominus.github.io/bootstrap-4/