A possibly related Q&A: Does halfling luck apply to death saving throws?
The description of the Bane spell reads (emphasis mine):
Whenever a target [...] makes an attack roll or saving throw before the spell ends, the target must roll a d4 and subtract the number rolled from the attack roll or saving throw.
The description of the bless spell reads similarly.
Based on this text, and the linked question, am I right to assume that these spells apply to death saving throws?