> Voter participation has almost monotonically increased over our country’s history [1], and presently stands near a historic high [2].
This is an insidious assertion, that is more about population increase than suffrage. Participation is always measured as a % by whomever is eligible, regardless of the political characterization of eligibility. The suffrage element incidental, rather than causal.
Populations have increased to historic highs, so "near historic high" participation is not interesting or relevant. As populations increase, the number of elections increase due to simple population bifurcation. eg City population grows, city splits in City and City-West and despite sharing utilities and proximity, there are now a bunch of new positions to elect for. Same thing happens when counties incorporate into formal cities due to growth. This has also added to "participation".
To limit the discussion from some of these factors, we can look solely at federal elections. That first wiki link displays incorrect totals, which can be verified multiple ways. ie Other wiki citations, the second link, etc
The increase of the total is not sloped upward, by % of the population. It's been roughly 50% with a variation of 10% regularly in the US, for all recorded data afaik.
https://ballotpedia.org/Voter_turnout_in_United_States_elect...
Edit: Had to edit as some of the language was inappropriate and overly broad, which did not add to the conversation.