Bitbucket Server/Data Center 7.13 (release notes) adds tags to the source selector when creating pull requests, and also enables the "Create pull request" button on the compare view when the source is a tag. When merging a pull request whose source is a tag, the "Delete ... after merging" checkbox will be disabled. Only source branches can be deleted on merge.
The target selector still does not allow tags, and on the compare view the "Create pull request" button is still disabled when the target is a tag. This will likely never change, because merging a pull request modifies the target and Git tags are intended to be immutable.
The source and target selectors still do not allow specific commits, and that's also unlikely to ever change. Pull requests generally include some amount of rework, which isn't possible if the pull request starts from a specific commit. Allowing it would also require substantial changes to both our API and the implementation code, both of which are heavily coupled to working with refs. Feel free to share use cases, if you have them, but the likely response will remain to either create a branch or tag on the commit you want to merge and use that.
Bryan Turner - Principal Developer
Bitbucket Data Center