If you would gzip your output like you should, how much does that even buy?
There's usually something better to use your time for instead of trying to shave 500 bytes out of your page.
it's not a competition, though. If there's something better to do, also do that. However, that does still leave the question of how many bytes are actually saved in transport, especially with gzipping. The benefit here is absolutely not individual developers or even individual sites, but the data transfered by entire data centers over the course of a day, week, month etc. If this recommendation can bring down the total byte transmission for "the web" by 0.001% for instance, that's still a boatload of bytes that don't bog down the network anymore.
When you're looking at fractions of a percent, remember to consider other options. Set up brotli, for example. Or redesign your site to have a leaner layout. You might not ever reach the efficiency level where optimizing optional tags is the best use of dev time.
And the overhead of tracking which tags are optional in which circumstances is not particularly small. Consider that the extra complexity could impede more optimizations in the future, especially now that your markup requires a more complex parser than it could have needed.
Have you looked at the size of Youtube and Netflix videos?
According to this study [1], 70% of web traffic is video streaming. Only 8% are web browsing (which might include images, because they are not mentioned anywhere else - didn't find any info on that).
Just because the vast majority of roads are for cars doesn't mean we should therefore not try to optimize the bike and pedestrian lanes.
Sure, a lot of the traffic is streamed data rather than HTML, but 30% of close to a zetabyte of data in a single day (for the internet as a whole) is still hundreds of petabytes that can be made drastically smaller. When the numbers are that large, even optimizing for something as "insignificant" as 0.01% of the traffic means 10s or even 100s of terabytes not pumped through the network every day.
As always: optimizations are not a matter a of "one or the other" they're about "do all of them". Make the build tool apply all optimizations and minifications, and make the client-server connection negotiate as much compression as possible. Don't stop at just gzip when further improvements are trivial (like this one).