If I were to leave the 401k as is:
No tax issues. If you are under age 55, then any withdrawals you make before age 59-1/2 will be subject to income tax plus a penalty. Some employers might kick you out of their plan especially if you have a low balance, you have to ask them. If they don't kick you out, you can leave it there indefinitely.
If I were to move the 401k to an IRA:
If you rollover your 401k to a traditional IRA, then the pro-rata rule would apply to any future Roth conversions -- it would make a true backdoor Roth IRA contribution impossible. That won't matter if your income allows direct Roth contributions, but might cause a problem if your income rises again.
The rollover is non-taxable. There is no difference between contributions and earnings since none were ever taxed, so there is no tax on a rollover. If you do a direct rollover from trustee to trustee, you might not have to report it at all (I forget this point) but even if you do get a 1099-R, you just report that fact and tell your tax software that it was a 100% rollover.
Some additional points to consider:
1. You should evaluate your investment choices. A 401k may offer limited investment options, but they may have lower expense ratios than buying the same fund through a private IRA. But an IRA may have more investment options.
2. Certain types of emergency withdrawals can be made from an IRA but not a 401k. But you can still leave the money in the 401k, and only rollover part of the funds that you need. (For example, I did a qualified HSA funding distribution this year to put $5000 into my HSA. That can only be done from an IRA, so first I rolled over $5K from my 403b to an IRA then rolled it over again from the IRA to the HSA. I left the rest of my money in the 401k because I am satisfied with the investment choices.)
https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-exceptions-to-tax-o...
