I target under 130 on brevets where I remember to bring both my HRM and my 2nd GPS that shows HR. I really like to roll along at 120; I *think* those numbers correlate to a sustainable level of effort for long durations, for me at my current level of fitness. That's a lot of caveats and weasel words.
As you stated, on many climbs that HR level is generally not possible even with lower-than-typical road bike gearing. Sometimes, exceeding my sustainable level of effort happens, and I'm okay dealing with the consequences. Brevets are not races.
As far as I'm concerned, the heuristic calculation is an approximation of what an average sedentary person's maximum HR will be. Other than making those of us who are not sedentary or not average feel better about ourselves because we beat the heuristic, it's pretty useless.
My max observed lately is 167, which at 61 puts me at 8 points above the heuristic. Go me
OTOH, that's lower than that claimed by a lot of my compatriots, so evidently I suck lol.