The sales support of (the consumer/prosumer sales subsidiary of) a DRAM manufacturer answered a pre-sales question I made with (translated from an email in French):
1.35V Memory DIMMs are dual voltage, and can work at both 1.35V et 1.5V; your computer system will adjust the voltage automatically as required by the motherboard and other factors like pre-existing DIMMs kept in the computer.
I was initially skeptical, but indeed the data sheet of even some relatively old DDR3L SDRAM chip states their are
Backward-compatible to VDD = VDDQ = 1.5V ±0.075V
Refer to the DDR3 (1.5V) SDRAM data sheet specifications when running in 1.5V compatible mode.
Is this the case for all DDR3L (1.35V) DIMMS on the market? If not, it is because they are unspecified, or without proper SPD memory content, or otherwise inoperative, or perhaps plain destroyed, when operated at 1.5V?
Note: This is an engineering question; I'm looking for technical details explaining incompatibility, if any. Like that pure speculation: for cost reason, some DDR3L chips are not tested at 1.5V (or happened to fail that test), and are thus sold on DDR3L 1.35V-only DIMMs not indicated as 1.5V-compatible in the SPD memory.