Can a married couple who have filed several years jointly, file separately even being still married?

[NewGuy]

Yes, you can always file either jointly or separately.

Married Filing Separately

You can choose married filing separately as your filing status if you are married. This filing status may benefit you if you want to be responsible only for your own tax or if it results in less tax than filing a joint return.

https://www.irs.gov/publications/p501/ar02.html#en_US_2016_publink1000220762