Are you talking about L.7? If so, it should be absolutely required, in the absence of such serious local problems that the Committee has a minuted decision that it's impractical. It's an RFU regulation:
Barrier from Field of Play
13.7.3 Each home Club playing a match in an RFU Competition must make provision:
(a) to ensure that (with the exception of Permitted Personnel as defined in 13.7.6 below) all other persons are kept at a reasonable distance from the playing area by permanent or temporary barriers and
(b) for two Technical Areas one for each team in a match to which Permitted Personnel (when not undertaking their duties) are to be restricted during the match other than for the period of half-time.
13.7.4 At Level 5 and below and only where the Club Committee is satisfied it is impractical or unnecessary for permanent or temporary pitchside barriers or Technical Areas to be in place each Club in a match must (unless the referee agrees otherwise) ensure that:
(a) with the exception of Permitted Personnel, all other persons remain at a reasonable distance from the playing area, and;
(b) Permitted Personnel (when not undertaking their duties) remain on the side of the pitch designated by the home Club within an area bounded by the centre and ten metre line on the pitch and two metres from the side line of the pitch at the end of the pitch in which it’s team plays the first half of the match.
13.7.5 Where there are permanent dugouts or shelters team replacements and coaching staffs must confine themselves to them or remain outside the pitchside barriers, or if none, the playing enclosure (as defined by the Referee) other than when a player is about to replace a player on the pitch, when a player is temporarily suspended or for the periods of half-time.
As a barrier can be a rope strung on some large pegs (as you often see around the cricket square), which can be achieved for a few hundred quid, surely the only reason to accept non-compliance at L.7 is a lack of physical space in which to erect it. If you have sucha lack of space, is it safe to play the game?