If they transmit crash logs, and those contain partial memory contents (core dumps), chances are actual content from network packets are transmitted to UBNT as well, and those contents may contain PII (IP addresses, MAC addresses, usernames, passwords, real names, addresses, credit card information, etc.), meaning they’re most certainly violating the GDPR by making it opt out.
The GDPR doesn’t care how you came by the PII, it only dictates how you should behave when you handle it.
Edit: I should clarify before somebody trips over their pitchfork and sets the house on fire, the GDPR cares a lot about how you come into possession of PII, and also if you should have that information in the first place, and also cares about how long you hold on to it.
When it comes to protection of those data though, it doesn’t care if it was collected through telemetry, emailed to you (yes, also the spam folder), or it’s an old archive of core dumps on an old server somewhere. The same rules apply to how those data are handled.