I dug deeper, and if you look at the catalog the installer is using: https://swscan.apple.com/content/catalogs/others/index-11-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog And search for "BridgeOSPredicateProductOrdering" you can see it only lists URLs for versions: .16610.0.0 .14538.0.0 .15580.0.0 You can see in the install logs where it is using this catalog to find the bridgeOS update: Nov 10 23:15:37 iMac-Pro osinstallersetupd[4758]: OSISDownloadOperation: Using catalog for bridgeOS: https://swscan.apple.com/content/catalogs/others/index-11-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog Nov 10 23:15:37 iMac-Pro osinstallersetupd[4758]: OSISDownloadOperation: bridgeOS update is required (minimum bridge version: 18.16.12561.0.0,0) Nov 10 23:15:41 iMac-Pro osinstallersetupd[4758]: Started downloading package com.apple.pkg.BridgeOSUpdateCustomer (http://swcdn.apple.com/content/downloads/41/51/001-73002-A_WRHOPHHLA3/wt9h9ieu492d5iadokij8dvq8t2dzg5sgg/BridgeOSUpdateCustomer.pkg) Nov 10 23:16:11 iMac-Pro osinstallersetupd[4758]: Retrieved package com.apple.pkg.BridgeOSUpdateCustomer (http://swcdn.apple.com/content/downloads/41/51/001-73002-A_WRHOPHHLA3/wt9h9ieu492d5iadokij8dvq8t2dzg5sgg/BridgeOSUpdateCustomer.pkg) Nov 10 23:16:11 iMac-Pro osinstallersetupd[4758]: Started downloading package com.apple.pkg.BridgeOSBrain (http://swcdn.apple.com/content/downloads/41/51/001-73002-A_WRHOPHHLA3/wt9h9ieu492d5iadokij8dvq8t2dzg5sgg/BridgeOSBrain.pkg) Nov 10 23:16:11 iMac-Pro osinstallersetupd[4758]: Retrieved package com.apple.pkg.BridgeOSBrain (http://swcdn.apple.com/content/downloads/41/51/001-73002-A_WRHOPHHLA3/wt9h9ieu492d5iadokij8dvq8t2dzg5sgg/BridgeOSBrain.pkg) Nov 10 23:16:11 iMac-Pro osinstallersetupd[4758]: Starting bridgeOS update prepare Nov 10 23:16:12 iMac-Pro osinstallersetupd[4758]: bridgeOS prepare summary: 17P6610 -> 17P6610, update = 17.16.16610.0.0, brain = 17.16.16610.0.0, variant = com.apple.bridgeOSCustomer, macOS = 19H15 (customer), project = BridgeOSInstall-80, userAuth = 0, prod fused = 1, model = j137ap, client = InstallAssistant, retries = 0, session = FC49A345-A2C3-4E90-AB0D-543B8B2FBFBD: Error Domain=BOSErrorDomain Code=110 "The bridgeOS update doesn't satisfy the given minimum version requirement." UserInfo={bridgeVersion=17.16.16610.0.0,0, minimumVersion=18.16.12561.0.0,0, catalogURL=https://swscan.apple.com/content/catalogs/others/index-11-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog, NSLocalizedDescription=The bridgeOS update doesn't satisfy the given minimum version requirement.} Nov 10 23:16:12 iMac-Pro osinstallersetupd[4758]: bridgeOS update prepare complete Nov 10 23:16:12 iMac-Pro osinstallersetupd[4758]: bridgeOS update prepare failed: Error Domain=BOSErrorDomain Code=110 "The bridgeOS update doesn't satisfy the given minimum version requirement." UserInfo={bridgeVersion=17.16.16610.0.0,0, minimumVersion=18.16.12561.0.0,0, catalogURL=https://swscan.apple.com/content/catalogs/others/index-11-10.15-10.14-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog, NSLocalizedDescription=The bridgeOS update doesn't satisfy the given minimum version requirement.} It seems to me the sucatalog on Apples end needs to be updated to reflect the URL of version 18.16.12561.0.0. I found some other posts online that indicate a "revive" of T2 as per https://support.apple.com/en-gb/guide/apple-configurator-2/apdebea5be51/mac can resolve it, but I have yet to test this myself.

Same on iMac Pro (2017)