After switching from Tiny Tiny RSS to Nextcloud News there was no possibility to filter items from my “unread” feed. But there is an API, so a quick Python script later, there was a filtering possibility (which even supports regular expressions). It doesn’t have a shiny UI, but a configuration file does the job just fine.
Just copy main.py and config-example.ini from this Github project, rename config-example.ini to config.ini, put your Nextcloud credentials and feed filters into it and create a cron entry (or append the script to Nextcloud’s cron entry), and you’re good to go. The readme file in the project has more details.