dn42 [0] also has, effectively, a playground for BGP. While I don't have experience with it personally, it seems to be just a VPN network with private addressing where you can find other folks to form neighbors with.
That being said, you don't need to be online to play with routing protocols. A couple VMs with your favourite flavour of Cisco/Juniper virtualised switches do just as well. Some of the most popular projects to simplify VM spin-up and interconnections are GNS3 [1] and EVE-NG [2].
[0] https://dn42.eu
[1] https://gns3.com
edit: link formatting