This is definitely true, although I find a good bit of variation in what people expect to be 'default play'. More people seem to know that Multiclassing is an optional module than that Feats are an optional module. Relatively few non-GM players are aware of DMG options, with the single strong exception of Milestone/ad hoc levelling - in some quarters I'd say that was seen as the default.
Because 5e is so flexible, the culture around 5e varies a lot from place to place. The ENW '5e culture' seems to emphasise a more 3e play style of build optimisation, for instance. I got a huge shock seeing Cody of Taking 20 talking as if it was normal for PCs to have 3e/4e style free purchase of DMG magic items. Elsewhere the 5e culture seems dominated by Critical Role and pastel-coloured PC sketches by nice young geek ladies.