I wouldn't call this a Pareto Frontier, but simple isolines through a 2d function.
There are weight x rep combinations that have a e1rm of 80kg, 85kg, 90kg, and so on.
These are just equal elevation contours through the e1rm(x, y) function.
The Pareto concept doesn't require that we calculate a function of all the dimensions and find contours; that sort of thing is not involved.
But we could apply it here like this. Suppose we conduct a weight lifting contest as follows: contestants can lift any weight any number of times, and record the weight and reps.
Then, how do we rank the results to find a winner, or winners? We have multiple dimensions, not a single dimension like "seconds to run 10 km".
We can find the Pareto front set of the performances by eliminating all that have been dominated. A lift is dominated if another lift is no worse (no less weight, and no fewer reps), and strictly better: eight the weight is higher, or there are more reps, or both.
We then end up with undominated winners, e.g. there could be three like this: { (100kg, 1), (80kg, 2), (70kg, 5) } but (70kg, 4) would not belong, due to being dominated by the third one, and (90kg, 1) would not due to being dominated by the first. The middle one is not dominated by either: though it's less weight than the 100kg, it is more reps, and though it is fewer reps than the 70kg, it is more weight.
Given the Pareto front set, if we want to determine a single winner, we need a function to reduce the parameters to a single value. (The function should be such that if we included the eliminated losers under that function, none of them would emerge winner over the Pareto front set). This e1rm function looks like it fits the bill.
If we have this function, we don't need the Pareto concept; we just run all the results through the function and pick the contestant(s) that maximize it.