I think a lot of that comes back to states deciding their own rules. I just realized I led you astray giving you good information about Wisconsin because every state is different. MN voter data, for example, is actually on their SOS website:
https://www.sos.state.mn.us/...oter-registration-counts/

Nevada doesnt post official voter data until after submitting official results because they allow same day registrations, but their updated daily total is kept here:
https://www.nvsos.gov/...home/showdocument?id=9054

PA DOS country registrations are here:
https://www.dos.pa.gov/...ngElectionStatistics.aspx

MN keeps it on their SOS here:
https://www.sos.state.mn.us/...oter-registration-counts/

NC keeps it on their SBOE website:
https://vt.ncsbe.gov/...ults/?date=11%2F03%2F2020

Wisconsin has theirs on their election commissions website here:
elections.wi.gov/node/7220

Michigan VIC has MI data compiled by SOS here:
https://mvic.sos.state.mi.us/VoterCount/Index

Arizona is the SOS website as well:
https://azsos.gov/...-historical-election-data

Georgia SOS as well, mind you their spreadsheet looks like hell and doesnt make sense to me, the last column in the race graph is total voters haha:
https://sos.ga.gov/...r_registration_statistics



Does that cover everything close? I did my best. My god do secretaries of state have some terrible looking bad websites tho lord.