Collecting rent will be removed. As of now to my knowledge there are no accounts anymore that are not rent exempt. When an account is rent exempt there is no rent collection anymore. This will enable that the feature will be completly removed from the code at some point.
You can see how much rent an account costs to be rent exempt using the command:
solana rent bytes
Here is the SIMD to remove rent collection: https://github.com/solana-foundation/solana-improvement-documents/pull/84
To find more details on how rent collection works/worked its probably easiest to read the issues or the code on the solana repository:
2020 you can see it was switched from lazy to eager collection for example: https://github.com/solana-labs/solana/issues/7413
Here you can see for example that the fees are collected as a blocking operation at the end of every block. https://github.com/solana-labs/solana/issues/18233
You can see rent collection caused quite some problems so that is one reason why it was removed: https://github.com/solana-labs/solana/issues/23342