"I'm not familiar with the direct share listing option, any thoughts on whether this could be better for slack?"

Ok to answer this I need to start on the differences between an IPO and a Direct Listing.

So an IPO is sort of a funding round + public listing. Meaning in most cases the issuing company issues new stock to be listed to be bought and the money collected goes towards investing in the issuing company. It is managed by an Underwriter (A major bank) that guarantees to purchase any stock that the market did not buy at a predetermined price. Being an investment round means most of the current investors in the company will not be able to divest their shares before the expiry of a lockup period.

A direct listing, on the other hand, means that the company is not using the listing to raise money and is allowing its current investors to divest their shares in the public market and profit instantly. This still allows Slack, in this case, to issue stock later on with the sole purpose of raising money but not on the date of listing. It also risks placing the company under pressure of market forces that do not set a minimum price at launch.

Now, which is better? It depends on your risk tolerance and your investors. Id your company is doing well enough that you are confident the market will buy when listed, a direct listing is a great way for the founders, investors, and employees to make money from the listing and then the company can choose to raise money from the market through a stock dilution or re-issuing. An IPO can guarantee a minimum amount raised but would not allow you as an investor to profit instantly.