Once upon a time a free theme was activated on one site and a lot of changes were made in it, so that someone would not accidentally update it and not overwrite the changes, I did the following:
In the menu “Appearance” – “Theme Editor” I selected the file “style.css” and changed the version, for example, it indicated “Version: 1.1”, and I indicated “Version: 99.1”. To apply the changes, I clicked the “Update file” button at the bottom.
In this case, if a theme update is released, for example version 1.2, then WordPress will not offer to update it since the version is installed much higher.
Before any such actions, I recommend making a backup copy of the site.
See also my article:
How to Disable Plugin Updates in WordPress