Remove nofollow attribute

That’s not the default behavior when adding links in post content. Are you using a different editor besides the default block editor? The attribute is likely added by a plugin or your theme (which may be where a different editor comes from). Narrow down which module it is by selectively deactivating plugins or switching themes. Then take up the issue in the dedicated support channel for that module.

Another approach would be to use “the_content” filter to strip out undesired attributes when the content is output. It’d be better to stop them from appearing to begin with.

I’m not sure how you investigated while in troubleshooting mode, a few factors can still confuse such an investigation. If the attributes are saved in post content, checking for their existence in output while troubleshooting would not reveal anything different. You may need to insert a new plain link and see that the attributes do not appear after saving. Even then, caching can confuse the issue further.

Troubleshooting mode while investigating carefully is the best way to narrow down the cause. Another way would be to use grep or similar search tool to find all instances of nofollow in your installation. Anything with the string could be the cause. WP has functions in formatting.php to insert the attribute, but they are not used by default.

If core WP really is inserting the attributes, you have a corrupted version of WP and I would recommend doing a manual update from a fresh download from this site.