I believe that many companies simply invent their own «Red Flags» when selecting candidates.
Working for many years in the 1st company? There will be disadvantages! Changing companies for your own development? Again, they will consider this a disadvantage. Unfortunately, now in the IT market, employers are trying to be not employers, but psychologists who are looking for only negative traits in a candidate, so for me, Red Flag is just such a company)
If the candidate knows his job and does it profitably, then it doesn’t matter how many companies he has changed or how long he has been in the same company. And if the employer does not evaluate the candidate based on his professional abilities, then I’m sorry, but this employer most likely has many problems of his own!
Therefore, choose good companies and good employers)