Interesting, it hadn't ever occurred to me that there's a use case for --keep-last-n and --keep-hourly etc in combination but this makes plenty of sense and support is relatively straightforward to add though it may take me a few versions to get to this. (edit: I'm assuming that you're coming from restic / know that these flags do actually work together -- I'll aim to test when I have some time).

Will track popularity of this FR in the meantime for my prioritization.

+1 for this feature, please.

Right now I can't use Backrest for anything more frequent than hourly backups, as the last ones just get pruned. :(

This command is an example of how I would do it on the cli, keeping all backups incl. ones that run every 15 min:

restic forget --keep-within 24h --keep-daily 90 --prune

First of all thanks, backrest is indeed pretty nice!.
This feature is very useful IMHO, keep-within the keep-last are cover very wide and useful combinations.
I would go even further. I would separate the forget command from the backup one.
Why would you run forget right after backup since forget does not reclaim/free space ?

In addition to this I think the retention policy explanation on the web interface is not correct. It says Snapshots older than the specified time period will be dropped. That would be the --keep-within-hourly --keep-within-daily etc. flags of restic.

However backrest is using the --keep-hourly etc. flags which keep the last x snapshots, only counting them if they exist for that time period.

For example if I specify Hourly: 48, expecting that hourly snapshots older than 48 hours would be dropped, but only do 4 snapshots on a day (separated by more than one hour), it will keep all snapshots for the last 12 days instead.