A digital game purchase is a licence to access, not ownership of a copy. Storefronts close, licences are revoked when publishers lose rights, and accounts can be lost. A few habits reduce the exposure.
What has actually happened
Storefronts shutting down, removing access to purchases for anyone who had not downloaded them.
Games delisted when music or likeness licences expired, remaining playable for owners and unavailable to new buyers.
Games requiring servers that were switched off, rendering them unplayable regardless of what you own.
What protects you
Download and keep installers where the platform allows it. Some storefronts provide standalone installers with no protection, which continue working regardless of what happens to the company. That is the strongest position available.
Account security. Losing your account loses everything in it. A strong unique password and hardware-based two-factor authentication are the whole defence.
Physical copies where the game is on the disc rather than a download code. Increasingly rare and genuinely durable.
What does not protect you
Downloading to a console, since the installed copy often still requires a licence check.
Assuming a large company will not shut down a service. Several have.
The realistic position
For most games, most of the time, digital access persists. The exposure is real and concentrated in specific cases – games with licensed content, games from small publishers, and games requiring online services.
Knowing which of your library falls into those categories tells you where to make an effort.