If you have left the Netherlands, you can still file Dutch tax returns, and it is often worth it. For the year you left, the M-form applies and usually produces a refund because wage tax was withheld as if you worked the full year. For earlier years in which you were a full-year resident, the Belastingdienst lets you file voluntarily up to five years back to claim refunds; Taxmo files the most recent completed of those, and older years go through Mijn Belastingdienst. You keep your BSN for life, so filing from abroad is possible.
The departure year is a migration year, so the M-form (M-aangifte) applies. Your employer withheld wage tax on annual tables while you only earned Dutch salary part of the year, which typically means a refund. The M-aangifte can be completed online in Mijn Belastingdienst with your DigiD.
Were you a full-year resident in earlier years and never filed? If you were not required to file, you may file voluntarily up to five years back. Deductions like mortgage interest, medical costs, donations, or a job change often mean the Belastingdienst owes you money for those years. Taxmo files these full-year returns directly from €49 for the most recent completed tax years; older years go through Mijn Belastingdienst.
Your BSN stays valid for life. Keep your DigiD active if you can (renewing it from abroad is harder), register a foreign bank account with the Belastingdienst for payouts, and keep your jaaropgaven. Refunds are paid out to foreign IBAN accounts as well.