Why not “I’ve spoken ... today”?

[alesult]

From Michael Swan's Practical English Usage:
'Definite' time-expressions (like today, this week, this morning) are not often used with the present perfect when we talk about finished events. Compare:
I've spoken to the boss about my holiday.
I spoke to the boss today about my holiday.

Why not “I’ve spoken ... today”?

[aaa]

'Cause you must not use PrPerf with certain time reference - i.e. "today"

[alesult]

That is not the real cause.
May be here the cause is in intonational emphasis?

I spoke ... today
But I’ve spoken ... today

[Tdol]

'Today' is a tricky time phrase when deciding between the past and poresent perfect. When it has the idea of 'so far today', then we can use the present perfect, but for a single finished event, the past is more common. In this example, I'd use the past tense, unless I had other things to talk about and were still at work. The second I left work, I'd automatically use the past for finished work activities.