For a long time.

There are two types of decentralization:

1. P2P - when there are no any servers at all. Shortly. it is like torrent with DHT or, say, eMule, but users send messages to each other, not files. Users find each other using DHT (Distributed Hash Table) - a shared database of user IDs. Example of P2P messenger is Tox. There are a lot of clients, most advanced, I think, is qTox. Since there are no servers, there is nothing to shut down. There is no possibility to censor it. Also, nobody will find you or your group without your will - best protection from shills. Users connects to each other directly, without any third-party. The disadvantage of P2P is a necessity to continously exchange with some data with other nodes to maintain a DHT. It is possible to use it on smartphone, but I will not recommend it, because you get fast battery discharge due to small, but permanent data exchange.

2. Federation. There is multiple servers, you could run your own, or, say, use the server of your trusted friend. Examples are e-mail, XMPP(aka Jabber), Fediverse (Mastodon, Diaspora, Frendica). Since there are no any central server, and servers could connect with each other, it can't be shut down easily. If you shut down one server, others will continue to work. The disadvantage is that you need a server. Run your own on VPS, or ask a trusted IT friend to set one up.

There are also IRC, ZeroNet, PeerTube and other projects.

Both variants have another huge advantage over centralized messengers - more users is better. :) Especially for P2P.

For a long time.

There are two types of decentralization:

1. P2P - when there are no any servers at all. Shortly. it is like torrent with DHT or, say, eMule, but users send messages to each other, not files. Users find each other using DHT (Distributed Hash Table) - a shared database of user IDs. Example of P2P messenger is Tox. There are a lot of clients, most advanced, I think, is qTox. Since there are no servers, there is nothing to shut down. There are no possibility to censor it. Also, nobody will find you or your group without your will - best protection from shills. Users connects to each other directly, without any third-party. The disadvantage of P2P is a necessity to continously exchange with some data with other nodes to maintain a DHT. It is possible to use it on smartphone, but I will not recommend it, because you get fast battery discharge due to small, but permanent data exchange.

2. Federation. There is multiple servers, you could run your own, or, say, use the server of your trusted friend. Examples are e-mail, XMPP(aka Jabber), Fediverse (Mastodon, Diaspora, Frendica). Since there are no any central server, and servers could connect with each other, it can't be shut down easily. If you shut down one server, others will continue to work. The disadvantage is that you need a server. Run your own on VPS, or ask a trusted IT friend to set one up.

There are also IRC, ZeroNet, PeerTube and other projects.

Both variants have another huge advantage over centralized messengers - more users is better. :) Especially for P2P.

For a long time.

There are two types of decentralization:

1. P2P - when there are no any servers at all. Shortly. it is like torrent with DHT or, say, eMule, but users send messages to each other, not files. Users find each other using DHT (Distributed Hash Table) - a shared database of user IDs. Example of P2P messenger is Tox. There are a lot of clients, most advanced, I think, is qTox. Since there are no servers, there is nothing to shut down. There are no possibility to censor it. Also, nobody will find you or your group without your will - best protection from shills. Users connects to each other directly, without any third-party. The disadvantage of P2P is a necessity to continously exchange with some data with other nodes to maintain a DHT. It is possible to use it on smartphone, but I will not recommend it, because you get fast battery discharge due to small, but permanent data exchange.

2. Federation. There is multiple servers, you could run your own, or, say, use the server of your trusted friend. Examples are e-mail, XMPP(aka Jaber), Fediverse (Mastodon, Diaspora, Frendica). Since there are no any central server, and servers could connect with each other, it can't be shut down easily. If you shut down one server, others will continue to work. The disadvantage is that you need a server. Run your own on VPS, or ask a trusted IT friend to set one up.

There are also IRC, ZeroNet, PeerTube and other projects.

Both variants have another huge advantage over centralized messengers - more users is better. :) Especially for P2P.