Is muscle isolation a good practice, bad practice or just not the most optimal?

There is no good or bad. It all depends on the situation.
Isolation is a misnomer: muscles all work together, synergy.
I use “isolation techniques” when I am teaching a cleint about certain muscle groups, but mostly as a tool not a technique of fitness.

Isometrics sometimes have to be used if a client has joint issues or mobility problems.

Personally I don’t see the point of isolating a muscle otherwise.