I have not tried this exact technique before, but if you don’t mind going one folder at a time, you could try the following method on Windows:
- Make sure you already have indexing options set to index .md files, which is not the default
- Open chosen folder
- Search for the tag
- Make sure the number of results matches how many you expect based on a search in Obsidian
- Select all files in the Windows Explorer search results and copy them
- Create a new folder and paste the copied files within
- Select all files in the new folder and copy them
- Paste the files into the original directory in your vault replacing existing (this automatically will highlight them)
- Delete the files
- Delete the folder created to temporarily store copied files
I am sure there is probably a much better way, but in the meantime I hope this helps.
Thanks.