Now we wait another five years for the ability to share deploy keys across repositories. If you have more than one project in your CI deployable app (for example a couple of internal python libraries), you can't use the same deploy keys. Their suggestion, "don't use modules, package everything in one application or use a full key". Now deploy keys can be R/O (fantastic), this limitation is double annoying.
Rather than using Deploy Keys at all it seems completely better in almost every way to create a fake GitHub user and use its account's regular SSH key. You can, using Teams, give that user read-only access to whichever repo(s) it needs access to for deployment.