Selection bias is also apparent in ranking sites for shows where seasons/sequels are ranked individually.

For example in anime, Gintama appears 8 times within the top 50: https://myanimelist.net/topanime.php

It's not because it's a popular show. It's just that people who didn't like the first few episodes have already stopped watching! And it's polarizing enough that the only people who stick around for so many seasons are the ones really love it. So it will get rated a 10 even for mediocre content.

True for a naive approach, but I'd think looking at the chart toppers for, say, each week would give decently comparable data.

I actually did something similar! I wrote a small blog post about it here https://www.popnalysis.com/blog/lyrics-over-time/

But I came to the same conclusion as the op comment. Music can't really be judged by any one metric! But that doesn't mean that you don't gain insight into how music has evolved!

also, I did release my entire lyrics dataset that I scraped (about 500k) for free.

Yes, assuming you can find all the old chart toppers (and that this term has a consistent meaning over the whole time interval you're looking at). If, understandably, you have some gaps in old chart toppers, you need to assume something about them (probably that they were worse than all surviving ones, which helps if you're doing percentile-based comparisons).