Realistically, only a few people have to do it and open source a toolkit. Also I don’t really think that we’ll have that issue with markdown because it’s widespread and rather well established. This doesn’t invalidate your point, which I absolutely agree with.
Also I would argue that you can rather simply write a parser for the basic markdown syntax and convert it to e.g. HTML or plain text if necessary by getting rid of markdown specific syntax.
Today I’ve learned! I knew about the export function but not that you could export the entire workspace. I’ll take a look into that. I’m currently 50/50 between Notion and Obsidian, however a blog post regarding obsidian plugins makes me think twice…
You might write your own Pipeline where you forward a mail, scrape the content and e.g. save it to a Markdown or plain text file to open it with your text editor of choice. I don’t know of any piece of software unfortunately.
What bugs me more than Notion not being open source is that the notes are nowhere on my device apparently but on their servers. I don’t have anything locally. If something happens to Notion or I don’t have access to the web, I’m done?
You might take a look at Obsidian. For me it’s the best of two (or how many you like) worlds. You can sync your files via git or a cloud service of your liking. What bugs me is that it’s not open source but what makes it great is a lot of open source developers contributing plugins and themes.
I‘m using DDG most of the time but when I have to use Google hoping the results will be better, I try to specify words that need to be in there. However, the more niche your problems are, the less you will find.