What are the implications of investing in a 401k as a non-US citizen who may eventually leave the country?

Benefits

Match - 401k contributions allow collecting employers' match. The match is not dependent on your immigration status, and is basically free money from your employer. This is a good incentive to go with 401k.

Tax deferral - Traditional 401k contributions, match, and Traditional IRA contributions (up to a certain level of income) don't count for income taxes when contributed, and taxed on withdrawal. Even if you withdraw them before the age of 60, as a non-US person you may find yourself in a much lower tax bracket in the US after returning home so that even the 10% penalty will make it worthwhile tax-wise.

Plans change - you may think you're not staying in the US, but in 20 years you may realize that you're actually a naturalized US citizen with family and kids here. It would be a bit late at that point to start accumulating retirement funds.

Issues to Consider

Tax at home - Taxation in your home country may be different than in the US. Your home country may not recognize the US retirement accounts as tax advantaged and you'll need to pay taxes on your gains/income in these accounts.

Expat support - Custodians may not allow foreign members. They may distribute the money to you (forcing a rollover and potentially creating a tax event for you) just because you're no longer a US person.

Foreign Accounts - some countries may not allow their residents to have accounts abroad, or regulate that. Check locally.

If you are a non-US citizen with a 401(k) and you think you might leave the US later, there are a few things to keep in mind in plain terms.

First, your 401(k) money grows tax-deferred, so you do not pay US tax now, that's the good part. But when you take it out in retirement, you will still owe US taxes on it as ordinary income, no matter where you live. If you live abroad, the IRS can withhold up to 30% on those withdrawals, unless your new country has a tax treaty with the US that lowers that rate (source: H&R Block - link you shared). Some countries tax that money again locally, so you might pay tax twice if there is no treaty or good rules to avoid it.

A Roth 401(k) or Roth IRA could make sense if you think you will be in a higher tax bracket later or if you want tax-free withdrawals. But to get tax-free treatment, you must follow all Roth rules; you need to hold it for at least five years and be over 59½ when you take the money out. Also, some countries do not recognise Roths as tax-free, so they might tax the growth anyway when you are living there.

A regular taxable brokerage account is more flexible, you can move it easily, no early withdrawal rules, and you only pay capital gains tax when you sell. But you do lose the upfront US tax break and employer match you get with a 401(k).

In your case, since you are under 30 and get a 50% match, it still usually makes sense to get the free money in the 401(k) that match is an instant return. But it is smart to mix in taxable or Roth investments too, so you do not get stuck with everything in a taxable-in-the-US bucket later.

The 401(k) match is worth taking now, but balance it with other accounts so you have options if you leave. And check if your future country has a tax treaty with the US that can really affect how much tax you’ll pay on withdrawals down the road.