90C is certainly a valid temperature. But as the auto manufacturers update parts and specs over the years, I wonder if they dropped the recommended operating temperature of their engines, and for what specific reasons?

This particular thermostat seems to be for early 90's to early 2000's ... do you think they are dropping the temp on the older engines as they age, as a longevity strategy?

Fyi I also checked out OEM units the later model years ... 19301-P8E-A10 is also 78C

 

Honda didn't change the specs. for a standard OEM thermostat.

The following are the specs. for just the ones I looked at; 5th, 6th, 7th, 8th generation Accords (other gens. may be the same too):

Starts opening: 76-80 C (169 - 176 F)
Fully opened: 90 C (194 F)