I was able to install the connector from http://update.atlassian.com/atlassian-eclipse-plugin/rest/e3.7 as suggested in:
- How to use Eclipse Neon's Mylyn with Jira since connector was discontinued
- How To Install Atlassian Connector On Eclipse Photon 4.8.x Version
Yet when I try to add a new repository it doesn't show Jira in there.

Is there a way to still make it work, maybe by downgrading mylyn?
There appears to be a fork here: https://github.com/rastaman/jira-mylyn-connector
With a single release in 2018 labelled:
I've not tried it yet as my setup using the details on this answer to get the last release from Atlassian, and Eclipse 2020-03 (4.15.0), was working with JIRA on premises.
However we've recently migrated to JIRA Cloud and although it connects I now get this error trying to synchronise task lists:
I assume this is some mismatch between fields in the connector and Jira Cloud so I'll report back here once I've had time to try the forked connector.