I know they are rarely used in outdoor fixtures like shoeboxes, post tops, flood lights, or cobra heads, but were they common indoors? Like in mall recessed cans, low bays, and high bays? Or did those mainly use the clear lamps too.

Because, I’ve seen both in the past, but were coated MH lamps ever common? I know coated HPS lamps are extremely rare, but what about coated metal halide.

They were less common than the clear ones, but they were still fairly common, especially indoors if the lamp was visible, such as high bay fixtures.

I’ve seen both clear and coated bulbs used indoors.

Those small wattage Osram HQI-E in fact are even easier to find coated than clear. Osram also did 400W coated elliptical HQI-E. Also BLV's elliptical lamps in 70-150W were also available coated. I am talking about European market, though.

Where I'm at (Metropolitan Detroit area of the United States), before the advent of LED everything, a lot of high bays had coated MH lamps, usually the ones with clear refractors rather than reflectors. It was a very common sight at various stores, warehouses, and gymnasiums. Another application where I saw a lot was medium base MH floods, usually for architectural lighting or flag poles, and the occasional FCO wallpack. Attached is a photo from the Google page of a now defunct thrift store that was lit by Coated MH lamps.

A lot of the low bays had them too. Not just high bays. Even the closed bottom ones frequently had coated bulbs (I’ve seen several closed bottom low bays with coated lamps based on the pattern on the refractor). Clear and coated lamps do not produce the same refractive patterns.

A lot of recessed cans also had coated lamps, and they’re also quite common in those recessed troffer fixtures and in canopy lights. I’ve occasionally seen them in shoeboxes, post tops, wall packs, or flood lights, though not as common. Coated metal halide is still way more common than coated HPS. Coated HPS bulbs were quite common in residential wall packs though, but that’s because that’s what they came with not because of user choice. Usually, once the lamp failed, the business (or consumer), would just put a clear one in. I’ve been to parking lots before that had all coated metal halide bulbs in the flood lights and shoeboxes.

It seems quite common for people to mistake coated metal halide for mercury vapor, which is why some people may think they’re rare. It’s quite easy to distinguish them especially when new. Coated metal halide lamps when new do not have as much pink in the spectrum as coated mercury vapor bulbs (coated MV lamps have a slight pink or purple hue to them that coated MH lamps don’t usually have when new). The CRI is also different (70 for coated MH vs 50 for coated MV).

Coated metal halide lamps were very common in gymnasiums, stores, warehouses, and in pretty much every place where MH is used indoors. Coated MH is much more common indoors than coated mercury vapor is. Coated MH lamps are easier on the eyes and are less glary which is why they were used so much in certain applications.

Indoor MV fixtures were widely converted to metal halide if installed. Indoor MV lighting was probably the first lighting to be converted to MH, and conversions often started in the 1970s or 1980s

When Fred Meyer started building larger "open ceiling" stores, they used metal halide high bays with coated lamps.  They seem to prefer warmer color temperature lighting, around 3000-3500K.  When the large store was built here in 1991, it was lit with probe start high bays with coated BT lamps.  During the 2001 remodel, they swapped to pulse start with coated ED lamps.  The second smaller store across town was built in 2005 with the same pulse start lights that were used at the other store after the remodel.

In 2008 they started replacing the metal halide lights at both stores with 6-lamp T8 fluorescent high bays.  At some point between 2015-2020, they were converted to LED.  I'm not as certain when that happened because that conversion wasn't as obvious since they just used InstantFit tubes.

Menards, Best Buy, and Lowe’s also all used coated lamps for interior store lighting if they used MH. Best Buy and Lowe’s eventually went fluorescent and then LED. Some Dick’s Sporting Goods locations also used coated MH bulbs (though some used clear). Toys R Us when it was open seemed to often use clear metal halide. Some locations had fluorescent.