Is showing transducer blocks on P&ID control valves an optional requirement?

There's no one right answer. Ideally the lead sheets for the site/plant would be followed by that's not always the case. Sometimes a certain project/area manager likes to show XYZ even though it's not consistent with other P&IDs.

Are you talking about the 'bubble' for the positioner or the rectangular block symbol that is attached to the stem of the actuator symbol that represents a positioner? P&ID's get closer to art than science, but PIP and ISA have these to say:

PIP PIC001 said:

4.5.3.5 Valve positioners shall not be shown unless necessary to clarify loop operation (e.g., if used with trip solenoids or pneumatic trip relays). Comment: If shown, valve positioners are normally included with the automated valve symbol and are not tagged.

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ISA 5.1 said:

B.5.2 It is not necessary to show a symbol or a bubble for every device or function required by a loop if the need for the device or function or its tag number is clearly understood; for example:
a) Symbols are not required, but may be used, for control valve positioners and stream sample conditioner components.
b) Bubbles are not required, but may be used, for orifice plate, thermocouple, and control valve graphic symbols.

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It comes down to whatever serves understanding, visual clarity, and consistency. FWIW I personally lean towards showing the symbol because I like establishing a clear visual distinction between modulating and on/off actuated valves.

I've seen some older drawings that show the I/P convertor, but all modern drawings dont show them, in my experience - its understood that it is part of the standard control valve setup in the field.

Such details are site specific and depend on a company practice, users are free to choose a suitable standard or create a new one. There is no a strict industry rule. For info see ISO 10628, ISO 14084, ISO 14617, ISO 15519, ANSI/ISA-5.1, PIP PIC001.

In general:
- if all CVs in this unit are fed with air then what is the reason to show this on P&ID? Distinction is reasonable only when a unit has different types of CVs
- if some CVs are fed with air, while other CVs with lets say oil then there is a reason to distinguish those to better understand which will fail during air pressure drop

Note that in the past a practice was widely spread to control CVs with impulse air, not 4-20 mA, because logic controllers were expensive and overcomplicated. So in the past a sound reason existed to distinguish I and P impulses and therefore how a particular CV is fed and how is controlled.

Note that at this time many units still have different feed sources at one time and place, e.g. as per my practice neighboring CVs may be fed with natural gas, inert gas and instrument air. In such case these details are critical for designers and operating personnel.

Note that some CVs (mainly depressurizing valves, shut-down valves, emergency emptying valves) have a built-in buffer and check valve intended for emergency supplying of gas during total pneumo failure. In this case it is critical to distinguish which CVs have extra safety devices.

Note that many processes may require some CVs to be fed with high pressure gas while the rest with common (low) pressure.