Bootstrapping P2P networks in a decentralized fashion is indeed a hard problem.
IPFS bootstraps by either having a static list of bootstrap peers (essentially normal nodes in the network) that are run by trusted organizations (currently some of them are run by Protocol Labs who also develop IPFS, libp2p and some other software), which is similar to how many P2P networks does bootstrapping. Or, if there is any nodes on the local network, those can also be discovered via mDNS that allows discovery on local networks. Once you've found one peer, you can exchange "address books" and find more peers.
Other ways, that I don't think are implemented anywhere in the wild as far as I know, is to do some sort of random-walk of the IP address space and just try to connect to random hosts, see if they are nodes. Obviously not super cheap, especially compared to bootstrap lists, and introduces some issues if the protocol is normally run on random ports and/or if NAT is being a particular bitch for the protocol to work around, which it tends to be. Coupled together with a local cache of previously seen nodes, it'll make initial bootstrap a bit slow but very resilient, and future disconnect/reconnects faster, but at least it'll be fully decentralized.
One could also imagine with the proliferation of new P2P technology like blockchains and eventually Matrix moving to P2P, that you might be able to use those for bootstrapping as well. If someone runs a Ethereum node with some Ether, maybe providing bootstrapping functionality for other networks could be something that could work via smart contracts? Just spit-balling here, but more out-of-band channels could be added for the discovery of peers.
Edit: a free-time curiosity of mine has always been to use inaudible sound for humans as a discovery protocol as well, but haven't really looked into that too much. If the nodes could use the microphone to pickup bytes from broadcasting nodes via audio, one might be able to put speakers in cities that aren't really noticeable for humans (but maybe too much for animals/other tech?) that allows nodes to bootstrap via it.
IPFS bootstraps by either having a static list of bootstrap peers (essentially normal nodes in the network) that are run by trusted organizations (currently some of them are run by Protocol Labs who also develop IPFS, libp2p and some other software), which is similar to how many P2P networks does bootstrapping. Or, if there is any nodes on the local network, those can also be discovered via mDNS that allows discovery on local networks. Once you've found one peer, you can exchange "address books" and find more peers.
Other ways, that I don't think are implemented anywhere in the wild as far as I know, is to do some sort of random-walk of the IP address space and just try to connect to random hosts, see if they are nodes. Obviously not super cheap, especially compared to bootstrap lists, and introduces some issues if the protocol is normally run on random ports and/or if NAT is being a particular bitch for the protocol to work around, which it tends to be. Coupled together with a local cache of previously seen nodes, it'll make initial bootstrap a bit slow but very resilient, and future disconnect/reconnects faster, but at least it'll be fully decentralized.
One could also imagine with the proliferation of new P2P technology like blockchains and eventually Matrix moving to P2P, that you might be able to use those for bootstrapping as well. If someone runs a Ethereum node with some Ether, maybe providing bootstrapping functionality for other networks could be something that could work via smart contracts? Just spit-balling here, but more out-of-band channels could be added for the discovery of peers.
Edit: a free-time curiosity of mine has always been to use inaudible sound for humans as a discovery protocol as well, but haven't really looked into that too much. If the nodes could use the microphone to pickup bytes from broadcasting nodes via audio, one might be able to put speakers in cities that aren't really noticeable for humans (but maybe too much for animals/other tech?) that allows nodes to bootstrap via it.