You can find a list of legacy installers compatible with macOS 10.6 and 10.7 / Windows Vista here:
Additionally, here is a list of legacy macOS installers for macOS 10.13 - 10.15 (latest 2023 releases):
ACME Opticom XLA-3: Download 1.10.0
A/DA Flanger: Download 1.5.0
A/DA STD-1 Stereo Tapped Delay: Download 1.5.1
ADPTR MetricAB: Download 1.4.0
AMEK EQ 200: Download 1.4.0
AMEK EQ 250: Download 1.1.0
AMEK Mastering Compressor: Download 1.1.0
Ampeg B15N: Download 1.5.0
Ampeg SVT3Pro: Download 1.5.0
Ampeg SVTVR: Download 1.5.0
Ampeg SVTVR Classic: Download 1.5.0
Ampeg V4B: Download 1.5.0
Bettermaker EQ232D: Download 1.0.2
Black Box Analog Design HG-2: Download 1.9.0
Black Box Analog Design HG-2MS: Download 1.3.0
bx_2098_eq: Download 1.8.0
bx_bassdude: Download 1.11.0
bx_blackdist2: Download 1.9.0
bx_bluechorus2: Download 1.11.0
bx_boom: Download 2.16.0
bx_cleansweep_pro: Download 1.10.0
bx_cleansweep_v2: Download 2.16.0
bx_clipper: Download 1.0.0
bx_console_amek_9099: Download 1.3.0
bx_console_focusrite_sc: Download 1.5.0
bx_console_n: Download 1.9.0
bx_console_ssl_4000_e: Download 1.7.0
bx_console_ssl_4000_g: Download 1.7.0
bx_console_ssl_9000_j: Download 1.4.0
bx_control_v2: Download 2.16.0
bx_crispytuner: Download 1.1.0
bx_delay2500: Download 1.8.0
bx_digital_v3: Download 3.10.0
bx_distorange: Download 1.11.0
bx_dyneq_v2: Download 2.16.0
bx_greenscreamer: Download 1.11.0
bx_hybrid_v2: Download 2.13.0
bx_limiter: Download 1.15.1
bx_limiter_true_peak: Download 1.2.0
bx_masterdesk: Download 1.7.0
bx_masterdesk_classic: Download 1.7.0
bx_masterdesk_pro: Download 1.1.0
bx_masterdesk_true_peak: Download 1.1.0
bx_megadual: Download 1.12.0
bx_megasingle: Download 1.12.0
bx_metal2: Download 1.9.0
bx_meter: Download 1.17.0
bx_oberhausen: Download 1.6.0
bx_opto: Download 1.10.0
bx_opto_pedal: Download 1.10.0
bx_paneq: Download 1.9.0
bx_refinement: Download 1.12.0
bx_rockergain100: Download 1.3.0
bx_rockrack_v3: Download 3.9.0
bx_rockrack_v3_player: Download 3.9.0
bx_rooms: Download 1.9.0
bx_saturator_v2: Download 2.12.0
bx_shredspread: Download 1.18.0
bx_solo: Download 1.16.0
bx_stereomaker: Download 1.14.0
bx_subfilter: Download 1.9.0
bx_subsynth: Download 1.9.0
bx_townhouse_buss_compressor: Download 1.8.0
bx_tuner: Download 1.11.0
bx_xl_v2: Download 2.16.0
bx_yellowdrive: Download 1.11.0
Chandler GAV19T: Download 1.11.0
Dangerous BAX EQ: Download 1.10.0
Diezel Herbert: Download 1.5.0
Diezel VH4: Download 1.5.0
Elysia Alpha Compressor: Download 1.17.0
Elysia Karacter: Download 1.9.0
Elysia Mpressor: Download 1.15.0
Elysia Museq: Download 1.13.0
Elysia Niveau Filter: Download 1.16.0
Elysia Nvelope: Download 1.10.0
ENGL E646 VS: Download 1.12.0
ENGL E765 RT: Download 1.11.0
ENGL Savage 120: Download 1.3.0
Friedman BE100: Download 1.4.0
Friedman Buxom Betty: Download 1.2.0
Friedman DS40: Download 1.4.0
Fuchs Overdrive Supreme 50: Download 1.3.0
Fuchs Train II: Download 1.3.0
Gallien Krueger 800RB: Download 1.3.0
Harris Doyle Natalus DSCEQ: Download 1.1.0
Knif Audio Knifonium: Download 1.4.0
Knif Audio Soma: Download 1.3.0
Looptrotter SA2RATE2: Download 1.1.0
Maag EQ2: Download 1.11.0
Maag EQ4: Download 1.15.0
Maag MAGNUM-K: Download 1.6.0
Mega Sampler: Download 1.1.0
Millennia NSEQ-2: Download 1.11.0
Millennia TCL-2: Download 1.11.0
Noveltech Character: Download 1.17.0
Noveltech Vocal Enhancer: Download 1.15.0
Pro Audio DSP DSM V3: Download 3.6.0
Purple Audio MC 77: Download 1.5.0
Shadow Hills Class A Mastering Comp: Download 1.4.0
Shadow Hills Mastering Compressor: Download 1.5.0
SPL Attacker Plus: Download 1.9.0
SPL De-Esser: Download 1.15.0
SPL De-Verb Plus: Download 1.9.0
SPL DrumXchanger: Download 1.15.0
SPL EQ Ranger Plus: Download 1.9.0
SPL Free Ranger: Download 1.18.0
SPL Hawkeye: Download 1.1.0
SPL Iron: Download 1.6.0
SPL Mo-Verb Plus: Download 1.9.0
SPL Passeq: Download 1.15.0
SPL PQ: Download 1.2.0
SPL Transient Designer Plus: Download 1.10.0
SPL TwinTube: Download 1.18.0
SPL Vitalizer MK2-T: Download 1.18.0
Suhr PT100: Download 1.2.0
Suhr SE100: Download 1.2.0
The Oven: Download 1.1.0
Tomo Audiolabs Lisa: Download 1.2.0
Unfiltered Audio LION: Download 1.5.0
Unfiltered Audio SpecOps: Download 1.3.1
Vertigo VSC-2: Download 1.15.0
Vertigo VSM-3: Download 1.10.0