Interesting development here, as the recall petitioners in the California Governor Gavin Newsom (D) recall are said to be soliciting signatures using direct mail. I'm not actually sure what that means. One critical part of the recall law is that the signatures must be witnessed by a circulator. Since this is part of an actual plan, I'm assuming that the recall petitioners and their lawyers have worked a way to do this without having the signatures tossed out in court.
This has been an issue in recalls elsewhere. The recall effort against the Governor of Alaska has also tried the mail-in route, to seemingly limited success. There was recent talk about a recall in New Jersey that would be signed online, though that seems based on an incorrect reading of the law.
As a comparison, the UK's new recall law requires petitioners to sign at one of 10 signing stations.
For Initiatives and Referendums, the National Conference of State Legislatures website claims that Oregon has found a way around witnessing for I&R, (but recalls require a direct witness, according to the Secretary of State's website). The NCSL also notes that "Florida law specifically permits the signing of petitions outside the presence of a circulator." Florida does not have a statewide recall law.