I have a question concerning this. I read that e.g. blockchain.info will "forget" (a.k.a. remove from mempool) a transaction after 48 hours, iirc.
That sounds about right. I'm not sure exactly what duration blockchain.info has chosen for their database, but it seems to be more than a day and less than a week.
Is that a special implementation at blockchain.info or is that something that applies to any wallet, no matter if it's on a mobile phone, a web wallet or a locally installed one on your PC/Notebook/Mac/whatever?
Each wallet handles unconfirmed transactions in its own way as determined by the person (or people) that created the wallet. As an example, Bitcoin Core never forgets a transaction
that it has sent (unless the user makes extraordinary effort to remove the transaction from the wallet. Instead, it just keeps on re-broadcasting the transaction every now and then in case any peers have forgotten about it. Most full nodes will hold unconfirmed transactions
that they have received for a while, but eventually they will remove them to make room for new transactions in the memory pool. The amount of time that the received transactions are held in the memory pool depends on the implementation, there are no protocol rules forcing any node to keep a transaction in their memory pool for any specific amount of time.
And, do such transactions indeed appear in the block explorers (as they were propagated to the network, already), stay in mempool and as soon as the period of 48 hours has passed, they disappear from the block explorers (if they were not included into a block)? Are they sort of "unpropagated" then, or do they have a flag set which "undoes" those transactions,
That's up to the people running the block explorer. Each block explorer can decide for themselves how they want to handle transactions that aren't confirmed after a significant amount of time. You'd have to list which explorers specifically you want to know about for anyone to be able to tell you what each one does.
or do they disappear forever from the blockchain then?
If they were unconfirmed, then they were never in the blockchain in the first place. Therefore, they can't "disappear forever
from the blockchain".