Written dress codes once specified what could be worn to particular events and workplaces with considerable precision. Most have become suggestions, and the change happened through several separate forces.
The workplace stopped enforcing a uniform
Office dress was for a long time effectively a uniform, maintained by supervision and by the visibility of everyone else wearing the same thing.
Sectors that competed for staff on culture rather than formality relaxed first, and the relaxation spread as those sectors grew in prominence.
Distributed and hybrid working removed the daily audience entirely, and a code that nobody observes being followed is difficult to sustain.
Comfort became an acceptable justification
Formal garments are built to hold a shape, which requires structure, and structure restricts movement to some degree.
As stretch fibres, knitted tailoring and softer construction became widely available, it became possible to look formal without the restriction.
Once the comfortable option was also presentable, the argument for the restrictive one had to rest on convention alone, which is a weaker position.
Events kept the language and lost the specifics
Invitations still carry terms like black tie or cocktail, but the shared understanding of what those require has thinned considerably.
Hosts increasingly add their own descriptions, which indicates that the term alone is no longer doing the work it once did.
The result is guests interpreting the same instruction across a broad range, and the range being socially acceptable in a way it previously was not.
Constant photography changed the calculation
When appearances at events are photographed and circulated widely, dressing identically to everyone present is a lost opportunity rather than a safe choice.
That pushes attendees toward distinctiveness, which necessarily means departing from whatever the code specifies.
Codes survive best where photography is discouraged or where the institution itself enforces them, which is a small and shrinking set of contexts.
What remains is signalling rather than rules
Dressing formally now communicates effort and respect for the occasion rather than compliance with a requirement.
That shifts the judgement from whether a garment is permitted to whether it reads as considered, which is a subtler and less teachable standard.
It also means the risk of getting it wrong has changed shape. Nobody is turned away, but the signal sent is still read by everyone present.