Past simple vs present perfect in popup

SHORT ANSWER

Consider an alternative option, of visually confirming the person's actions. Do guide the person from one action to the next ('Proceed to your workspace?'), but avoid extraneous options (Will you check the bin?).

LONG ANSWER

First, do know that in design some things are a matter of preference. Here the difference between the efficacy of both proposed solutions (was vs has been) is minor. Had it been major (for example: Data was/will be imported), choosing one or the other would be less a matter of preference and more right or wrong (in the example, 'will be' implies that the action has not been done yet, so unless this is accurate, 'was' would be the 'right' choice).

Next, often times when creating interfaces, 'right' tends towards what is conventional (as it will be familiar for the user) and what is easy (so it can be designed/implemented quickly). However, this does not necessarily lead us to a great design. Great design sometimes challenges convention AND is often a pain in the butt to implement. It is rooted less in what other people typically design (bad/lazy) and more in an understanding of the task at hand and human psychology.

Pop ups are both conventional and easy to implement, but often not the best from the user's perspective (think of all of those websites that while you are scrolling a popup appears asking for your email).

From your question it seems to me like you are trying to confirm to the user an action occurred AND usher them in to a likely next action.

A good interface requires no extra effort. To confirm that an action has taken place, think about where the person is looking. Assuming the user just pressed a button with an icon and text, we know the person is looking at this button. Could you confirm the action by changing the icon to a check mark and the text to a confirming word (like 'deleted')? Then after a second, animate away the confirming elements?

Here are some more things to think about, but they are speculative as I don't know the product you are working on:

• Data was/has been imported. Will you go to your workspace?

If the dominant use case is to import data and then go use it in the workspace, just take the person to the workspace showing them the imported data (skipping the popup box). Also, when we create things, we give them names, but this jargon is often confusing to users. If you can get the user to use the product effectively without knowing what a workspace is, great. That is preferred. Think of the number of products you've used that have some terminology for some part of their product and you don't know WTF they are talking about.. Of course, I can envision situations where you need to teach the user that this is a workspace and it's imperative to your product as well.

• The files were/have been deleted. Will you check the bin?

No.

When you interrupt a user with a question with 2 options, the utility of each option should be roughly 50%. This is the reason why you are asking, because you literally have no idea which option they want and it's imperative that they tell you. Here, very few people when deleting something want to go look at it in the bin. You can think of the proportion of people hitting the 'yes check bin' button being something like 1%. Since this option is rarely chosen, prompting the person should be avoided, and rather just make it obvious where it went (maybe again through animation) so if they want to retrieve it they can.. or offer an undo button.

Material Design discourages the use of the present perfect tense and encourages the use of simple word forms (see section Write in the present tense).

So I would either go for

Data imported. Will you go to your workspace?

or

Data was imported. Will you go to your workspace?