The issue is that the company can track what you are reading.
Then you have to decide for yourself if you're OK with that or not.
I know Microsoft looks at my files when I put them on my OneDrive. I take that into account when I decide what to put there. I know Google reads my mail when Thunderbird sends it through their SMTP servers. I know it'll be read by some unknown parties along the way to the recipient. I take that into account when I write my mail.
If I pasted a URL into a feed reader, I'd most certainly assume the app, and by extension its creators, would access that URL and read what's there. I'd take that into account when using the app.
Then I wouldn't be surprised.
I haven't read Thunderbirds privacy policy in detail, so for all I know there's a clause about collecting email addresses in there. Could even be for a good reason, spam filtering for example.
And as mentioned I take that into account when writing emails. I do indeed avoid writing certain stuff and avoid sending certain mails due to privacy. For the rest I see it as an acceptable risk/reward tradeoff.
I'm not a tinfoil hat kinda guy, but the innocence is gone when it comes to modern software.