Most of the witnesses from #55 and below are 'former' witnesses that are running old witness versions (0.23.0 or 0.22.1) and have now moved to another blockchain.

Very right! But unfortunately, some of these witnesses still appear in various lists.

I have noticed that, for the sake of simplicity, only the signing key is checked. If this is the null key, the witness is marked as inactive in the lists. But the null key is mainly intended for temporary manual deactivation of the witness node. In the case that a witness has not produced any blocks for some time, this witness would also have to be deactivated. This is however not reflected in the various lists (e.g. gtg (#57), scissor-sisters (#33)).
In my opinion, unfortunately, only the page https://steemitwallet.com/~witnesses still contains an up-to-date and correct listing of active witnesses.

update
The steemitwallet additionally checks whether a block has been produced in the last 7 days. The witness is shown deactivated if the null key is deposited or no block has been produced in the last 7 days:

const noBlock7days = (head_block - lastBlock) * 3 > 604800;
const isDisabled =
    signingKey == DISABLED_SIGNING_KEY || noBlock7days;

^{Code aus steemwallet}

update 2
I wondered why witnesses who have obviously not generated blocks for a long time are no longer automatically shut down.
After a bit of research, I found the answer:
The automatic deactivation described in the Steem whitepaper and introduced with Hardfork 14 was removed again with Hardfork 20. Here is the detailed conversation about it: https://github.com/steemit/steem/pull/2835

Here my appeal is once again very appropriate to check the witness votes regularly!