It should be noted that the developers of these applications usually do so altruistically on a non-profit basis.
A rating system can lead to toxicity or simply undeserved negativity.
It’s not really necessary.
This is not the same as the apps of those other stores where they are commercial either directly or indirectly with a multitude of trackers and ads.
I’m not saying that there are no paid apps on F-Droid or that it’s wrong for it to be that way.
For inexpensive things, like apps, it usually takes less time and money to get it and try it yourself, than to read through reviews and decide which reviews and ratings can be trusted, which are fake pro or con reviews, or are by people who have completely different values in apps.
If it’s important to you, a web search for detailed reviews can sometimes be more efficient, and may even find discussions in this forum.
As someone who publishes to both F-Droid and Google Play, I can’t count the amount of times I feel regret for having spent any time coding because of certain people giving 1 star reviews on Google Play because one minor thing isn’t as they want and being extremely rude about it.
I understand ratings can help to give a better idea of what you’re going into, but the way it works on Google Play is EXTREMELY demotivating for developers like me.
Consolidated the topic 
One should read FaireMail/Netguard’s developer posts elsewhere on rating and fairness, it’s really sad.
I’m not saying everything should be “5 stars 'cause it’s free” but it should be judged in the context of FOSS. Everyone likes constructive criticism, but that’s hard to come by. Also it’s hard to come by the acceptance of stuff not being fixed/added every time one complains, as everyone work on what they like in the end.
One should read FaireMail/Netguard’s developer posts elsewhere on rating and fairness, it’s really sad.
One should give a link. The controversial history is sadder, and probably not what you intended us to find. 
I loled
IRL? I LOL IRL at people reading and commenting 8 year old open issues. 
To summarize, ratings are over-rated.
Hard, bits and pieces, eg. [CLOSED][APP][5.0+] FairEmail - Fully featured, open source, privacy oriented email app | Page 78 | XDA Forums
or
[CLOSED][APP][5.0+] FairEmail - Fully featured, open source, privacy oriented email app | Page 79 | XDA Forums
via
Paid features in opensource apps
I remember that fight now. But honestly, most stuffs on XDA go like that. When a user requests, and as mentioned dev goes haywire, it turns to much towards hatred and fiery words. I was have a hearty laugh out of it.
Was not a fight, we understand that devs get pressured by low Play ratings
Paid features in opensource apps
TL;DR all, but it has one of the worst and best statements I’ve read recently:
since basically everyone in the world could afford 1 Euro a year to use software.
Things change, but to me if remotely accurate, these indicate 1 Euro a year is still a high price, on top of basic costs of a phone, for many people in the world.
I still think F-Droid should require all users to click through a payment prompt, with a default value set by the developer, and the option to set it to 0.
I agree, and should cancel my F-Droid Liberapay until implemented., to encourage it. 
Although F-Droid is doing well on Liberapay now, I feel the reliance on donations, from day 1, has been a mistake. Implementing a payment system, with default prices set by developers (and small “tax” taken by F-Droid and processor friends), would be a great service, and big improvement over the “click to buy a coffee” links.
How is gnu Taler going BTW? I still can’t get anything over Tor from their repo.
GNU Taler… wonder if the current versions are any newer, 2020 is soooo long ago.
Can comments be possible. Like Google play with easy commenting opinion and rating of an app. People could even use the same account of this forum