Perhaps you should make sure the content isn't already online somewhere? Archive.org has an extensive archive of BYTE, for instance.
https://archive.org/details/byte-magazine

There was some excitement recently when archive.org started hosting an extensive archive of old issues of Nintendo Power – but they took it down after some kind of DMCA notice. I'm a bit fuzzy on the details (and there have also been questions of whether every legal action related to Nintendo does in fact necessarily have any real connection to Nintendo).

Didn't know that the existed, not a fan of their layout though and their missing a lot of the older maximum pc magazines that I have.

No, it is not legal unless the publisher (or the owner of the rights) entitles you to do so.

Microhobby (a Spanish ZX Spectrum related magazine) is legally available because they contacted with the owner and were granted the rights to publish. Note that it means that a single site is entitled and other people can't (again: unless entitlement the copyright owner) publish those files legally.

Other magazines are published with licenses that grant the right to reproduce their contents, but they are few.

Wouldn't it be great to have the old "phone book" sized Computer Shopper as an archive. Every month was a 2lb+ book of prices for computer parts (mainly X86 PC). I wonder if Ziff-Davis would give permission to archive those, but who would want to do it. At its height, I think it went over 800 pages for nearly two years.

I think some scanners would break apart after a few of those doorstops have been done, and would require the effort of a robot to scan them seriously