I would agree, but is that because we have been playing in a lesser competition than we have historically ?
(TL;DR - Not really).
Here's our record since Advocaat took over.
1998-99 - UEFA 3rd Round (last 16) - knocked out v Parma - 2 Quali/2 rounds before this
1999-00 - UEFA 3rd Round (last 32) - dropped out of CL/knocked out v Dortmund
2000-01 - UEFA 3rd Round (last 32) - dropped out of CL/knocked out v Kaiserslautern
2001-02 - UEFA 4th Round (last 16) - knocked out CL Qualifiers/knocked out v Feyenoord
2002-03 - UEFA 1st Round (last 96) - direct entry / knocked out by Viktoria Zizkof (shudder)
2003-04 - CL Group Stage - 1 qualifying round / finished bottom of group
2004-05 - UEFA Group Stage (next stage would be last 32) - knocked out CL Qualifiers/finished 4th of 5 in the group
2005-06 - CL Round of 16 - 1 qualifying round/qualified from group/knocked out by Villareal
2006-07 - UEFA 4th Round (last 16) - entered at first round, through groups to 3rd round/knocked out by Osasuna
2007-08 - UEFA Final - 2 CL Qualifiers, dropped into UEFA / lost final to Zenit
2008-09 - CL Qualifier Round 2 - knocked out by Kaunas
2009-10 - CL Group Stage - direct entry / finished bottom of group
2010-11 - Europa League Round of 16 - direct entry to CL group, dropped to UEL / knocked out by PSV
2011-12 - Europa League Playoff - knocked out of CL playoff round by Malmo / knocked out of EL Playoff round by Maribor
2012-13 - nil
2013-14 - nil
2014-15 - nil
2015-16 - nil
2016-17 - nil
2017-18 - Europa League 1st Qualifying Round - knocked out by Progres Niederkorn
2018-19 - Europa League Group Stage - 4 qualifying rounds / finished 3rd in group
2019-20 - Europa League Last 16 - 4 qualifying rounds / finished 2nd in group / through 1 knockout round / knocked out by Bayer Leverkusen
2020-21 - Europa League Last 16 - 3 qualifying rounds / finished 1st in group / through 1 knockout round / knocked out by Slavia Prague
Apart from under McLeish in 2003/04 when we got through the CL qualifiers 2005/06 when we got through the CL group into CL last 16, and in 2008 when we got to the UEFA Cup final, we haven't made it past a CL qualifier or the last 16 of the UEFA / Europa League, so we're pretty much finding the same level / stage of competition as we have over the past 20+ years.
There's an argument that the teams were of a higher standard in the early 00's because CL teams dropping out were better, but now the financial disparity/gap in technical ability compared to other leagues has grown and Scotland lags well behind so it is harder for us to keep getting to these stages now.
I think to rebuild from virtually no European pedigree in 2018 to winning the group in 2020 is a phenomenal achievement given where we are financially. Say what you want about Gerrard but the elite culture and winning mentality he brought to us has paid off at that level and the consistency of results against very good teams like Legia, Feyenoord, Porto, Braga, Galatasaray, Benfica, Liege and Antwerp (some CL level teams) speaks for itself.
Progress was probably inevitable, but I think if you'd said to anyone after our disaster to Progres Niederkorn that within 3 seasons we'd be topping our group and getting further in Europe than Celtic you'd have been laughed out of town.