With the release today of Plasma 5.21.90 (5.22 Beta), testing for the next major Plasma version has started. Final release is planned for three weeks from now.
As always, place kde-next above all in your pacman.conf. If you do not know how to edit pacman.conf, then please don't try to attempt testing betas of Plasma, editing the pacman.conf is the easiest part of this kind of testing.
New added are ksystemstats & layer-shell-qt. Ksysguard no longer is included in the regular Plasma releases (it still gets a standalone update, at least this time).