You can insert comments in a style file using the tag src-comment. This tag is available when you use the source style—you have to select that style in addition to naming your style file with the .ts extension. You get a pastel yellow box framed with a thin black line.
When using the source style you see the assign tags even if you are in text mode, i.e. Document->Source->Edit source tree is unchecked. (edited 2023-12-29, see next post for correct version)
In source style you have available as well the tag src-title, which is used by TeXmacs to check whether a style is being applied to the document, see incomplete details here:
This is wrong.
Here is a better version.
You have to be in source mode editing to see by default the assign tags, and you can activate the comments and title by selecting them with the cursor and then using the menu to add the active* tag:
The other way round, i.e. not being in source mode and deactivating the assign tags to see them, might not work as you want the assign tags to assign. I did not try this.