I am currently playing with a game that was last played uder 1.4. I had stopped playin and just recently came back under 1.7

In that save, many horses had died, including shadowmere. Now I find it strange to find all my dead horses resurected, including shadowmere. I found him exactly on the spot he died months ago, torn to pieces by a foresworn ravager.

1-Is that a "glitch" introduced by the new patches or is that an intended feature?

2-If shadowmere dies again, will he resurect again or is that only a one shot deal?

Shadowmere will respawn 15 days after he dies, at the location where he died. This has always been the case, as far as I know, so I don't think it is a bug.