Since automatic relegation was introduced, here's who has gone down from the league and how many seasons they spent out before returning or not:
86-87 Lincoln City (1 season)
87-88 Newport County (25 seasons but were dissolved and had to reform)
88-89 Darlington (2 seasons)
89-90 Colchester United (2 seasons)
90-91 no relegation but we finished bottom
91-92 Aldershot were expelled (16 seasons but were dissolved and had to reform)
92-93 Halifax Town (5 seasons)
93-94 no relegation
94-95 no relegation
95-96 no relegation
96-97 Hereford United (9 seasons)
97-98 Doncaster Rovers (5 seasons)
98-99 Scarborough (not returned but have had to reform and were never a league club prior to 1987)
99-00
Chester (4 seasons)
00-01 Barnet (4 seasons)
01-02 Halifax (not returned but had to reform)
02-03 Exeter (5 seasons) and Shrewsbury (1 season)
03-04 Carlisle (1 season) and York (8 seasons)
04-05 Kiddy (not returned but not a league club prior to 2000) and Cambridge (9 seasons)
05-06 Rushden (not returned but were dissolved and reformed and not a league club prior to 2001) and Oxford (4 seasons)
06-07 Boston (not returned but not a league club prior to 2002) and Torquay (2 seasons)
07-08 Mansfield (5 seasons) and us (no need to say we haven't yet returned)
08-09
Chester (not returned but were dissolved and reformed) and Luton (5 seasons)
09-10 Grimsby (6 seasons) and Darlington (not returned but were dissolved and reformed)
10-11 Lincoln (6 seasons) and Stockport (not returned)
11-12 Hereford (not returned were dissolved and reformed) and Macclesfield (6 seasons)
12-13 Barnet (2 seasons) and Aldershot (not returned)
13-14 Bristol Rovers (1 season) and Torquay (not returned)
14-15 Cheltenham (1 season) and Tranmere (3 seasons)
15-16 Dagenham (not returned) and York (not returned)
16-17 Hartlepool (not returned) and Leyton Orient (2 seasons)
17-18 Barnet (not returned) and Chesterfield (not returned)
So out of all the teams that have been out of the league for more seasons than us, Newport, Aldershot and Halifax all had to reform after being disolved; Kiddy and Boston were not league teams prior to the introduction of automatic promotion/relegation and Rushden and Scarborough were not league teams before automatic promotion/relegation was introduced and also were wound up and reformed.