Stainless steel extinguishers are still manufactured sold and are legal.
All new models must be CE marked by an approved body and meet the Pressure Equipment Directive.
They are made to the construction & performance requirements of BS EN 3, but cannot be kitemarked if polished bare metal as they are not red, which is a requirement of the standard. The same applies to "Stainless steel effect" CO2 extinguishers (actually polished aluminium cylinders).
Stainless steel extinguishers do exist as kitemarked models - obviously older models to BS5423 could be & were kitemarked, but new extinguishers also exist with BSEN3 kitemarking - these however are painted red. Amerex water & foam extinguishers always have and continue to be made of stainless steel and the red painted models are BSEN3 kitemarked (BS7937 for the Wet Chemical model).
If the manufacturer is approved body certified, BAFE accredited, etc then this can be used as contributory evidence in lieu of the actual product being kitemarked
