The recommendations for this almost always say not to bother with anything more than .com/.net/.org and the ccTLD for your country. I would never register multiple domains for anything where I don't have the .com.
However, I think there are situations where you can get good value for your money in terms of brand protection. The sentiment I usually see is that no one's going to build anything using a "bad" TLD, but there's no agreement on what's "bad". I personally consider .io to be bad for a tech business, but obviously there are a lot of successful startups that don't agree with that opinion.
In the context of brand protection, I try to think about the likelihood of someone else starting to using the same name as me because they're able to get a "good" domain. Since "good" is subjective, it's important to consider options that you think are "bad" because there's likely someone that has a different viewpoint and might settle for a domain you wouldn't.
I think .io is a perfect example of this. If you read Hacker News or anything in the tech startup space, there's a healthy debate about the strategy of securing a .com immediately vs saving that capital by using a lesser TLD like .io until you know you have a viable product. Regarless of what anyone thinks is right, lots of startups settle for .io.
So, if you register a tech sounding .com, I think .io should be high on the list of domains to register for brand protection. I might even rank it as the 2nd priority after .com because even though .io has less market share than something like .org, I assume the odds of a tech startup settling for .io to be higher than those willing to settle for .org. That might be wrong because it's not based on any real data.
Regardless, I think you can get extremely good value for your money by registering alternate TLDs if you're doing it for brand protection. Compared to something like trademark protection, the costs are minimal and the outcomes are predictable with global effects. For example, by registering a .io you deprive everyone else in the world from that domain and it has the effect of devaluing that name from the perspective of any tech startups because they're more likely to judge the remaining available domains as "bad".
Since "bad" is subjective, the difficult part is figuring out the point where you stop getting good value for the money you're spending. I think it makes the most sense for moderately generic names. If your name is "store" it's too generic for registering multiple domains to be an effective strategy because you can't register in enough TLDs to make a difference. If your name is "ryanshomeautomationtech", it's specific enough that you probably don't need more than .com.
However, if your name is "gluetech" and you're an influencer making your living from tutorials about glueing things, it's going to suck if a tech startup building a no-code product registers and popularizes gluetech.io. Even worse, that startup is going to be registering social media handles that overlap with your name. You might have @-gluetech, but the startup is still going to usurp search results with @-gogluetech or @-officialgluetech. You'll also be in different industries, so a trademark probably doesn't help much.
You need to judge it on a case-by-case basis, but in scenarios like the one above, I think it makes a lot of sense. I have a domain like that for a project I want to build and I spend about $250 USD per year to keep the name registered in (what I think are) the top 12-15 most viable TLDs.
because the Name "Google" is protected.
