IPA becoming unobtainium, can I use denatured alcohol to clean vintage computers?
The stuff that's sold as "denatured rubbing alcohol" here in the US is typically ethanol (90%) and methanol (10%). I would be concerned about using it to clean boards: methyl alcohol will attack some plastics.

"Denatured alcohol" like what you've shown might have other stuff in it, depending on the source. Besides methanol, this can include pyridine, benzene, acetone, MEK, isobutyl ketone, and possibly others. Again, not knowing the components on your board, these denaturants could damage it.

Here's some background on solvents from Chemtronics: https://www.chemtronics.com/Content/Images/uploaded/documents/2005V4N5HydrocarbonsvsChlorinated.pdf

You're in Tennessee, you can get 190 proof Everclear which would do the job without risking damage from methanol.