I believe that 2021 IRC Section M1502.4.5 is new, but M1502.4.4 is not. That said, I don't believe that booster fans were ever truly permissible. Their listings were not inclusive of dryer exhaust duct systems. Not saying that folks were aware of that, but I do not believe that the 2021 code is a substantial change. DEDPV were already the correct piece of equipment, the 2021 just spelled it out further because of folks not reading the listings.
In the 2018 IRC, the path to requiring DEDPV was not as clear cut, but it was still there. Booster fans should never have been permitted. Unfortunately, the path to get there did not spell it out explicitly, so some folks did not enforce it appropriately.
The path, via the 2018 and prior is as follows. Via M1502.4.4, DEDPV must conform to UL 705. Via M1502.4.5, three options are provided for determining the allowable length of a dryer exhaust duct. 35-ft rule, manufacturer's instructions, or a DEDPV. Those are the three options. A domestic booster fan is not one of the permitted options; therefore, not permitted.
2018 IRC
M1502.4.4 Dryer Exhaust Duct Power Ventilators
Domestic dryer exhaust duct power ventilators shall conform to UL 705 for use in dryer exhaust duct systems. The dryer exhaust duct power ventilator shall be installed in accordance with the manufacturer's instructions.
M1502.4.5 Duct Length
The maximum allowable exhaust duct length shall be determined by one of the methods specified in Sections M1502.4.5.1 through M1502.4.5.3.
M1502.4.5.1 Specified Length
The maximum length of the exhaust duct shall be 35 feet (10 668 mm) from the connection to the transition duct from the dryer to the outlet terminal. Where fittings are used, the maximum length of the exhaust duct shall be reduced in accordance with Table M1502.4.5.1. The maximum length of the exhaust duct does not include the transition duct.
TABLE M1502.4.5.1 {omitted for clarity}
M1502.4.5.2 Manufacturer's Instructions
The size and maximum length of the exhaust duct shall be determined by the dryer manufacturer's installation instructions. The code official shall be provided with a copy of the installation instructions for the make and model of the dryer at the concealment inspection. In the absence of fitting equivalent length calculations from the clothes dryer manufacturer, Table M1502.4.5.1 shall be used.
M1502.4.5.3 Dryer Exhaust Duct Power Ventilator
The maximum length of the exhaust duct shall be determined in accordance with the manufacturer's instructions for the dryer exhaust duct power ventilator.
