> Bitcoin is an open source permissionless protocol, so you have multiple clients to chose from, each with their own list of bootstrapping nodes, many open source where you can submit a PR to add your node too. You can even build your own client and point to whatever you want. You can also just ignore them and just point directly to nodes in a list from a public forum, a private chat, whatever.
I characterized this as relying on centralized authorities (albeit several of them), but sure, it can also be considered decentralized to some extent.
The point is that it's a mechanism outside of the proof-of-work network itself. Instead of relying on a machine to reach consensus via a formal protocol, you the human are probing for a social consensus by evaluating statements made by other humans (via GitHub, public forums, or private chats, or just talking to people in person).
In both proof-of-work and proof-of-stake, you need to find social consensus in order to initially obtain the software, after which point you can rely on the network's consensus.
The difference with proof-of-stake is that you have to redo this if you disconnect from the network for months on end.
In practice, for a variety of reasons, practically all users of cryptocurrencies download regular software updates, and thus continue to rely on social consensus, regardless of whether the currency is proof-of-work or proof-of-stake.
