First of all, the old version of Electrum has always been available either on
GitHub:
https://github.com/spesmilo/electrum/releasesor official website:
https://download.electrum.org/so you can download and use those if you wish.
Will Electrum stay backward compatible?
So far the new versions have always been backward compatible meaning they recognize the older version of wallet.dat and/seed seed and can convert it to the newer version.
I was using electrum 2.6.4, and my question is will it stay backward compatible?
I am not referring to the wallet format, but the seed & master priv key and master public key structure? Will a new electrum say 3.x.x be able to recognize the seed of a 2.6.4 wallet?
So far it does. And for the future, it should continue doing so.
Will the electrum servers load up a 2.6.4 wallet say 1 year from now?
I don't understand what you mean by this.