As Rick notes, a cert. petition has been filed asking the Supreme Court to hold unconstitutional Texas’ law that permits all those — and only those — 65 and older to vote absentee for any reason. That petition implicates in part the Supreme Court’s major precedent on absentee voting, the 1969 decision in McDonald v. Board of Election Commissioners of Chicago. There is not a lot of election-law scholarship on McDonald, partly because absentee voting has not occupied the central role in most states that it will this fall. One of the more extended discussions (a highly critical one) I’m aware of can be found in this piece by Justin Driver.