Ay, PureOS may not be recognized as a distro to add a repo, but the bigger problem to me seems to be that https://packagecloud.io/app/crowdsec/crowdsec/search?dist=&filter=all&page=3&q=aarch lists aarch64 (as in, compatible with L5 architecture) only for rpm packages, not dep. Contact Crowdsec to hear if they would be willing to update and include PureOS with aarch64. Of course, you could try using https://github.com/crowdsecurity/crowdsec and make from source.
Edit: for anyone interested, intro to Crowdsec: (“CrowdSec is not a SIEM, storing your logs (neither locally nor remotely). Your data are analyzed locally and forgotten.
Signals sent to the curation platform are limited to the very strict minimum: IP, Scenario, Timestamp. They are only used to allow the system to spot new rogue IPs, and rule out false positives or poisoning attempts.”). Also: https://docs.crowdsec.net/docs/intro/