Even if they’re not looking at revisions, all table of contents plugins are likely to only look at post_content rather than postmeta, because postmeta can be anything and it would be hard to query for something that doesn’t exist on every site.
Your best bet may be to update your site to use the Block Editor. From there, you can create a custom block that displays the postmeta within the post_content. Then, some plugins may be able to pick up the heading tags – though some may still not, so you might want to first experiment with some other blocks that create heading tags that are not the Core Heading block to find a TOC plugin that picks those up.