Google places very little weight on expired domains. Its not ilegal and you wont get penalised but it wont increase your SEO either. Pretty useless technique.

It is not worth it because i really doubt it will do your seo any good, do not try take shortcuts when it comes to seo because your website will be at risk to be penalized.

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Nothing is illegal in SEO

It is definitely legal in the sense of the law and not against ToS of Google to redirect a domain to your website. Redirecting a domain to your website will not just potentially increase your websites stats and ranking but could also increase the amount of traffic heading to your site.

Any traffic that is visiting the domain you want to redirect will end up on your website so if it is a related site you will gain in that respect and this could improve your site stats alone. If the domain is related and it has not been spammed or penalised in any way then it will not pass on any negativity to your site either, in fact if it hasn't got any negativity attached to it and is related it likely will help the ranking of your website.

All of this is dependant on the site being very related to the original topic of the domain.

If the domain is not at all related then you can make it related by putting up some related content and then placing just 1 or 2 links from that website (the one on the domain you were considering redirecting) to your money site and this will be the start of your own private PBN that will help increase your rankings.

There are many uses of expired domains and you would think from reading here that they should be avoided like the plague when in reality a lot of the uses for them are not against any ToS